Help with Marco

I have the code below to put an “a” in column A when you click in it, and to take it out if you click again, but it is doing away with the undo function, is there any way to change the code so this will not happen? Thanks

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
Dim rng As Range, cell As Range
If Selection.Cells.Count = 1 And _
Not Intersect(Selection, Columns(1)) Is Nothing And _
Not Intersect(Selection, ActiveSheet.UsedRange) Is Nothing Then
With Selection
If .Value = "a" Then
.ClearContents
Else: .Value = "a"
End If
End With
End If
End Sub